1. Field of the Invention
This invention is a corrugated plastic article and, more particularly, in a prime embodiment, is directed to the use of such article as a protective member for the core of an electrical cable.
2. Description of the Prior Art
The use of plastic film as a protective member in electrical cables is old in the art. The film is wrapped around the outer surface of the core and, thus positioned, provides considerable protection for the core. Polyester film, such as polyethylene terephthalate film, available commercially under the trademark "Mylar", owned by E. I. du Pont de Nemours and Company, is particularly effective in providing various types of protection.
In this connection, the protective member must provide not only electrical and physical separation of the core from an intermediate metallic shell or other elements of the cable in the finished state, but must withstand the rigors and stresses resulting from heat and other factors during use and manufacture of the cable.
In order to meet these multiple requirements, it often has been found necessary or advisable to use various types of and configurations (i.e., corrugated) of protective materials. As an example U.S. Pat. No. 3,244,799 to Roberts shows the use of a single corrugated plastic film as a core protective means or member in a typical illustration of the prior art teachings.
Such corrugated plastic film provides a low-cost thermal barrier for the core while additionally providing the familiar excellent abrasion resistance, puncture resistance, high melting point, chemical resistance and dielectric strength properties of such film. Through the use of this corrugated plastic film, the excellent balance of properties continues to aid cable manufacturers in lowering reject rates and manufacturing costs while maintaining high reliability in the finished cable.
Many corrugated protective members of this type, however, tend to "wilt" under high heat use or manufacturing conditions, thus causing the protective member to lose its required corrugated shape and to function in a less desirable manner, particularly as a cushioning device and as insulation, due to the diminished height (measured from corrugation peaks) of the protective member.
In known cable wrapping applications, the film used is relatively thin (2 to 3 mils thick prior to corrugating, for example) and is corrugated under heat and pressure in the lengthwise or machine direction of the film to a corrugated thickness of around 9.5 mils. This thin single layer of film is not always suitable for insulation in extreme, high-heat situations as it lacks the required dielectric strength and mechanical strength and shape-retention capabilities in this particular use situation. One purpose of the corrugations of the protective member is to provide as much air space as possible to improve thermal insulation. To do so requires that the protective member retain its basic shape. This shape-retention capability of applicants' corrugated, multiple layer protective member is the keystone of this invention.
It has been found that the corrugations of the protective member, if of proper height and shape, provide effective and critical air spaces in the area between the core and the intermediate shield and outer covering or jacket of the cable. These air spaces provide pneumatic cushioning against mechanical forces which might otherwise damage the core and protection also is afforded by the resilient nature of the corrugations themselves. Inasmuch as the plastic material is substantially non-absorbent, damage by moisture which might change the electrical properties of the windings is avoided.
The following, then, are among the basic property requirements sought in a protective member or wrap for the core of an electrical cable: (1) resists chemical attack, (2) good dielectric strength, (3) electrical stability, (4) good insulating properties, (6) rigidity, (7) good mechanical strength, (8) good shape retention capabilities, and (9) good cushioning properties.
The corrugated plastic protective member or article of this invention, by retaining its essential shape, even under extreme conditions, provides improved cushioning and strength and further provides improved thermal insulation and dielectric strength in meeting the above-named requirements.
It is evident, of course, that the corrugated plastic article of this invention has other uses than as protective material or member in cables. Its novel shape-retention properties give it wide utility, as readily will be understood, and this invention includes that utility.